1. Field of the Invention
The present invention generally relates to laser scanning systems for reading symbols such as bar code symbols and, more particularly, to a lightweight, multi-component, portable laser diode scanning head supportable by a user and aimable at each symbol to be read. Still more particularly, this invention relates to a light focusing arrangement and method in which a light beam is separated into a first part useful for scanning a symbol to be read, and a second part useful for illuminating an aiming region on the symbol.
2. Description of the Prior Art
Various optical readers and optical scanning systems have been developed heretofore to optically read bar code symbols applied to objects in order to identify the object by optically reading the symbol thereon. The bar code symbol itself is a coded pattern comprised of a series of bars of various widths, and spaced apart from one another to bound spaces of various widths, said bars and spaces having different light-reflecting characteristics. The readers and scanning systems electro-optically decoded the coded pattern to a multiple alpha-numerical digit representation descriptive of the object. Scanning systems of this general type have been disclosed, for example, in U.S. Pat. Nos. 4,251,798; 4,360,798; 4,369,361; 4,387,297; 4,409,470 and 4,460,120, all of which have been assigned to the same assignee as the instant application.
As disclosed in some of the above patents, a particularly advantageous embodiment of such a scanning system resided, inter alia, in emitting a laser light beam from a hand-held, portable laser scanning head which was supported by a user, aiming the head and, more particularly, the laser light beam, at a symbol to be read, repetitively scanning the laser beam in a series of scans across the symbol, detecting the scanned laser light which is reflected off the symbol, and decoding the detected reflected light. Inasmuch as the laser light beam was usually, but not always, generated by a helium-neon gas laser which emitted red laser light at a wavelength of about 6328 Angstrom units, the red laser light was visible to the user and, thus, the user, without difficulty, could properly aim the head and position and maintain the emitted red laser light on and across the symbol during the scanning.
However, in the event that the laser light beam was generated by a semiconductor laser diode, as by way of example, see U.S. Pat. Nos. 4,397,297; 4,409,480 and 4,460,120, then the aiming of the head relative to the symbol was rendered more difficult when the laser diode emitted laser light which was not readily visible to the user. For some laser diodes, the laser light was emitted at a wavelength of about 7800 Angstrom units, which was very close to infrared light and was on the borderline of being visible. This laser diode light was visible to the user in a darkened room, but not in a lit environment where ambient light tended to mask out the laser diode light. Furthermore, if the laser diode light was moving, for example, by being swept across the symbol, and especially if the laser diode light was being swept at fast rates of speed on the order of a plurality of times per second, for example, at a rate of 40 scans per second, then the laser diode light was not visible to the user, even in a darkened room. Hence, due to one or more of such factors as the wavelength of the laser light, the intensity of the laser light, the intensity of the ambient light in the environment in which the laser light was operating, the scanning rate, as well as other factors, the laser diode light was rendered, in effect, xe2x80x9cinvisiblexe2x80x9d, or, as alternately defined herein and in the claims, as being xe2x80x9cnon-readily visiblexe2x80x9d.
This non-readily-visible laser diode light did not enable the user, however, to readily aim the laser diode light at the symbol, at least not without a great deal of difficulty and practiced effort because, simply put, the user could not see the laser diode light. The user, therefore, was required to hunt around by trial and error, hope that the scanning laser diode light was eventually properly positioned on and across the symbol, and wait until the scanning system advised him, typically by the lighting of an indicator lamp or by the sounding of an auditory beeper, that the symbol had indeed been successfully decoded and read. This hunting technique was a less-than-efficient and time-consuming procedure for reading symbols, particularly in those applications where a multitude of symbols had to be read every hour and every day.
Nevertheless, in the context of a laser scanning head which was desired to be made as lightweight, miniature, efficient, inexpensive and easy to use as possible, the laser diode was more advantageous than the helium-neon gas laser, despite the non-readily-visible laser diode light characteristic, because the laser diodes were smaller, were lighter in weight, had reduced power requirements (voltage supplies on the order of 12v DC or less), were directly modulated for synchronous detection and for increased signal-to-noise ratios, etc., as compared to such gas lasers.
However, despite the above advantages, certain optical properties of the laser diode beam itself, aside from its invisibility, did not readily enable the laser diode beam to be focused to a desired spot size (e.g. a 6 to 12 mils circular spot) at a given reference plane exteriorly of the head, and to maintain said spot size within specified tolerances at either side of the reference plane within a predetermined depth of focus or field, i.e. the working distance in which a symbol located anywhere within the field can be successfully decoded and read. For example, the longer wavelength of the laser diode beam, as compared to that of the helium-neon gas laser dictated a shorter working distance for the same spot size. The laser diode beam was also highly divergent, diverged differently in different planes, and was non-radially symmetrical. Thus, whereas the gas laser beam had the same small divergence angle of about one milliradian in all planes perpendicular to the longitudinal direction of beam propagation, the laser diode beam had a large divergence angle of about 200 milliradians in the plane parallel to the p-n junction plane of the diode, and a different larger divergence angle of about 600 milliradians in the plane perpendicular to the p-n junction. In the single transverse mode (TEM00), the gas laser beam had a radially symmetrical, generally circular cross-section, whereas the laser diode beam had a non-radially-symmetrical, generally oval cross-section.
By way of example, in a so-called geometrical approach to solve the aforementioned focusing problem, and ignoring the non-radially symmetrical nature of the laser diode beam, optical magnification factors in excess of 80 were obtained if one wished to focus the beam spot to have about a 9.5 mil spot diameter at a reference plane located about 3xc2xdxe2x80x3 from the head. However, such high magnification factors dictated that, if one optical focusing element were employed (e.g. see U.S. Pat. No. 4,409,470), it would have to be critically manufactured, positioned and adjusted. If one employed several optical focusing elements in a lens system designed with a large numerical aperture, i.e. on the order of 0.25, as suggested by U.S. Pat. No. 4,387,297 to accept a large divergent laser diode beam and to distribute the magnification among the elements, then the mechanical tolerances for each element would be looser, and the positioning and adjustment procedures would be easier. However, a multiple, as opposed to a single, optical element design occupied more space within, and increased the weight and expense, of the head.
Also, although an oval laser diode beam spot was, in certain cases, desirable in ignoring voids in, and dust on, the symbol, as well as in rendering the light-dark transitions more abrupt, as compared to a circular gas laser beam spot during a scan across a symbol, these advantageous features occurred when the longer dimension of the oval spot was aligned along the height of the symbol. Thus, to obtain such desirable features, the laser diode beam had to be correctly aligned in a certain orientation relative to the symbol. In a situation where the symbols were oriented in a random manner relative to the laser diode beam, the head had to be frequently manipulated to correctly orient the laser diode beam on the symbol, and this further aggravated the already less-than-efficient and time-consuming procedure for reading symbols, particularly on a mass basis, with laser diode light. Although it was possible to circularize the oval laser diode beam spot using an anamorphic collimator, this further increased the number of optical elements, the space, the weight and the expense.
Still another drawback inherent in earlier laser scanning heads, both of the gas laser and laser diode type, was that they were not readily adaptable to different applications. Different end users had different requirements. Whereas one user might want the electronic circuitry for decoding the detected reflected light to data descriptive of the symbol, and for controlling the decoding, to be mounted in the head, another user would require this electronic circuitry to be located remotely from the head. Still other users had different requirements concerning whether or not to locate a rechargeable power source or a data storage either locally in, or remote from, the head. Thus, the prior laser scanning systems had, more or less, to be custom-made for each user, and this was not altogether desirable in terms of manufacturing or marketing. Also, if the user wished to change the system requirements, the user had to forego the change, or obtain another system.
Yet another disadvantage associated with earlier laser scanning heads was that each had a discrete light-transmissive window mounted thereon. The discrete window was a separate piece which had to be glued in place and, hence, over time, the window sometimes became free of its glued mounting, particularly if the head was frequently subjected to mechanical shock and abuse. Once the window became disengaged, moisture, dust and other such contaminants were free to enter the interior of the head, thereby coating the optics and the electronic circuitry therein, and possibly interfering with their intended operation.
1. Objects of the Invention
It is a general object of this invention to overcome the above-described drawbacks of the prior art laser scanning systems.
It is another object of this invention to enable a user to readily aim a head and, more particularly, to direct at a symbol a non-readily-visible laser light beam emitted from the head at, and/or to collect non-readily-visible reflected laser light reflected from, the symbol.
It is a further object of this invention to enable a user to readily aim a non-readily-visible laser beam emitted by a semiconductor laser diode on and across a symbol prior to and during a scan of the symbol.
Yet another object of this invention is to eliminate the trial-and-error hunting techniques, particularly at long working distances, in aiming a semiconductor laser diode beam at a symbol.
Still another object of this invention is to increase the efficiency and reduce the time involved in optically reading a symbol with a semiconductor laser diode beam.
A still further object of this invention is to accurately locate a symbol with a semiconductor laser diode-based scanner prior to a scan, and to accurately track the symbol with the semiconductor laser diode-based scanner during the scan.
Another object of the invention is to readily enable a highly divergent, non-radially symmetrical, long wavelength, semiconductor laser diode beam having a generally oval beam cross-section to be focused to a desired beam spot size at a given reference plane exteriorly of the head, and maintained at said spot size within specified tolerances at either side of the reference plane within a predetermined depth of field without requiring a single high-precision, high-magnification optical focusing element to be manufactured or precisely positioned relative to the diode, and without requiring multiple optical focusing elements to occupy increased space within the head.
Another object of the invention is to provide an efficient and compact optical folded path assembly within the head having a novel optical element for reflecting an aiming light beam, but for transmitting a semiconductor laser diode beam, and also having a novel one-piece multi-purpose scanning mirror, collecting mirror and focusing mirror.
A further object of the invention is to provide a multi-position, manually-depressible trigger for controlling the operation of an aiming light arrangement, as well as that of the laser scanning system.
Still another object of the invention is to provide a modular design for the components in the head, wherein different components are receivable in either a single handle, or in readily interchangeable handles, mounted on the head, for readily adapting the head to different requirements of different users.
Yet another object of the invention is to provide a very lightweight, streamlined, compact, hand-held, fully portable, easy-to-manipulate, non-arm-and-wrist-fatiguing laser diode scanning head and/or system supportable entirely by a user during the optical reading of symbols, especially black and white symbols used in industrial applications, but also bar code symbols of the type known as the Universal Product code (UPC).
An additional object of the invention is to seal the interior of a head from contaminants.
Another object of the invention is to use one part of the laser beam for scanning, and another part of the laser beam for aiming.
2. Features of the Invention
In keeping with these objects and others which will become apparent hereinafter, one feature of the invention resides, briefly stated, in an aiming light arrangement for use in aiming a hand-held laser scanning head in a laser scanning system for reading symbols at which the head is aimed. Several components are conventionally mounted in the head. For example, means, e.g. a semiconductor laser diode or possibly a gas laser, are provided within the head for generating an incident laser beam. Optic means, e.g. a positive lens, a negative lens, reflecting mirrors, or other optical elements, are also provided within the head for optically modifying, i.e. forming, and directing the incident laser beam along a first optical path toward a reference plane located exteriorly of the head and lying in a plane generally perpendicular to the direction of propagation of the incident laser beam, and to a symbol located in a working distance range in the vicinity of the reference plane. For convenience, a symbol that is located between the reference plane and the head is defined hereinafter as a xe2x80x9cclose-inxe2x80x9d symbol, whereas a symbol that is located on the other side of the reference plane away from the head is defined as a xe2x80x9cfar-outxe2x80x9d symbol.
Laser light is reflected off the symbol, and at least a returning portion of said reflected laser light travels along a second optical path away from the symbol back toward the head. Scanning means, e.g. a scanning motor having a reciprocally-oscillatable output shaft on which a reflecting surface such as a scanning mirror is mounted, are mounted in the head for scanning the symbol in a scan, and preferably at a plurality of sweeps per second, across the symbol in a repetitive manner. The returning portion of the reflected laser light has a variable light intensity across the symbol during the scan which is due, in the case of a bar code symbol, to the different light-reflective characteristics of the bars and spaces which constitute the symbol.
The head also comprises sensor means, e.g. one or more photodiodes, for detecting the variable light intensity of the returning portion of the reflected laser light over a field of view, and for generating an electrical analog signal indicative of the detected variable light intensity. Signal processing means are also mounted in the head for processing the analog electrical signal, and usually for processing the analog signal to a digitized electrical signal, which can be decoded to data descriptive of the symbol being scanned. The scanning means is operative for scanning either the incident laser beam itself across the symbol, or the field of view of the sensor means, or both.
Sometimes, but not always, decode/control electronic circuitry is provided locally in, or remotely from, the head. The decode/control electronic circuitry is operative for decoding the digitized signal to the aforementioned data, for determining a successful decoding of the symbol, and for terminating the reading of the symbol upon the determination of the successful decoding thereof. The reading is initiated by actuation of a manually-actuatable trigger means provided on the head, and operatively connected to, and operative for actuating, the laser beam generating means, scanning means, sensor means, signal processing means, and decode/control means. The trigger means is actuated once for each symbol, each symbol in its respective turn. In a preferred embodiment, the actuation of the trigger causes the actuation of the decode/control means which, in turn, causes the actuation of the laser beam generating means, scanning means, sensor means and signal processing means.
In conventional usage, the head, which is supported by a user in his or her hand, is aimed at each symbol to be read, and once the symbol is located, the user actuates the trigger means to initiate the reading. The decode/control means automatically alerts the user when the symbol has been read so that the user can turn his or her attention to the next symbol, and repeat the reading procedure.
As noted above, a problem arises when the incident laser beam or the reflected laser light are not readily visible, which can occur due to one or more of such factors as the wavelength of the laser light, the laser light intensity, the ambient light intensity, the scanning rate, as well as other factors. Due to such xe2x80x9cinvisibilityxe2x80x9d, the user cannot see the laser beam and does not know readily when the invisible laser beam is positioned on the symbol, or whether the scanning laser beam is scanning over the entire length of the symbol.
Hence, in accordance with this invention, the aiming light arrangement assists the user visually to locate, and aim the head at, each symbol when such non-readily-visible laser light is employed. The aiming light arrangement includes means including an actuatable aiming. light source, e.g. a visible light emitting diode, mounted in the head, and operatively connected to the trigger means, and operative, when actuated by the trigger means, for generating an aiming light beam whose light is readily visible to the user; and aiming means, also mounted in the head, for directing the aiming light beam along an aiming light path from the aiming light source toward the reference plane and to each symbol in turn, visibly illuminating at least a part of the respective symbol and thereby locating the latter for the user. The aiming light path lies within, and preferably extends parallel to, either the first optical path or the second optical path, or both, in the portion of such paths which lie exteriorly of the head. Thus, the user is assisted in correctly aiming the head at the respective symbol to be read.
In one advantageous embodiment, the aiming light arrangement directs a single aiming light beam to each symbol to illuminate thereon a generally circular spot-region within the field of view, and preferably near the center of the symbol. It is further advantageous if this single spot region remains stationary or static during the scanning of the symbol so that both close-in and far-out symbols can be seen and located by the user, both prior to and during the scan. However, one drawback associated with such static single beam aiming is that the user cannot track the linear scan of the scanning beam across the symbol during the scan. In other words, the user does not know where the ends of the laser scans are and, hence, does not know whether the linear scan is extending across the entire length of the symbol, or is tilted relative thereto.
In another advantageous -embodiment, the aiming light arrangement directs a pair of aiming light beams to each symbol to illuminate thereon a pair of generally circular spot regions that are within, and spaced apart of each other along, the field of view. Preferably, the two spot regions are located at, or near, the ends of the linear scan, as well as remaining stationary or static during the scanning of the symbol so that both close-in and far-out symbols not only can be seen and located by the user both prior to and during the scan, but also can be tracked during the scan. However, one drawback associated with such static twin beam aiming is that two aiming light sources and associated optics are required, and this represents increased system complexity, weight, size and expense.
In still another advantageous embodiment, the aiming light arrangement directs a single aiming light beam to a reciprocally oscillating focusing mirror operative to sweep the aiming light across each symbol to illuminate thereon a line region extending along the field of view. Such dynamic single beam aiming is advantageous because close-in symbols can be more readily seen, located and tracked, as compared to static aiming. However, one drawback associated with such dynamic aiming is that far-out symbols cannot readily be seen, located or tracked, particularly when the focusing mirror is being swept at high scan rates on the order of 40 scans per second due to the inherently reduced intensity of the light collected by the human eye.
In a further advantageous embodiment, the aiming light arrangement directs a single aiming light beam to a focusing mirror which has a stationary state and a reciprocally oscillating state. Initially, the aiming light beam is reflected off the stationary focusing mirror to each symbol to illuminate thereon a spot region within the field of view, preferably near the center of the symbol, prior to the scan of the symbol to locate the same. Thereupon, the focusing mirror is caused to reciprocally oscillate to reflect the aiming light beam to the symbol to sweep the aiming light beam across the symbol to illuminate thereon a line region extending along the field of view, thereby tracking the symbol. This combination static/dynamic aiming is very desirable because it enables a user to track a close-in symbol during the scan (which was not readily possible with only static single beam aiming), and also enables the user to at least locate a far-out symbol prior to the scan (which was not readily possible with only dynamic aiming). Since, in the majority of cases, the symbols to be read will be close-in symbols, the inability to track the far-out symbol in the combination static-dynamic aiming embodiment is not critical.
To implement such combination static/dynamic aiming, it is advantageous if the trigger means has multiple positions and is operatively connected, either directly or indirectly via the decode/control means, to the aiming light source, as well as the oscillatable focusing mirror. In a first position, or off state, for the trigger, all of the components in the head are deactivated. In a second position, or first operational state, the aiming light source is activated, and the focusing mirror is positioned in a predetermined stationary position, e.g. in a center position, for a predetermined time, so that the aiming beam can illuminate a center spot region of the symbol to be read. In a third position, or second operational state, all of the other components in the head, including those responsible for reciprocally oscillating the focusing mirror, are activated, thereby initiating the reading of the symbol and the illumination of a line region along the field of view.
All of the above aiming light arrangement embodiments are in direct contrast to those that were provided on wand or pen readers which were manually positioned on, or at a small distance from, a symbol, and thereupon which were manually dragged or moved across the symbol. Skilled users were generally required to effect the aforementioned movement because criticality in the manipulation of the angle of the pen relative to the symbol, the pen speed, the uniformity of the pen speed, and other factors was necessary. In any event, the manual reader only results, at best, in one scan per manual movement and, if the symbol was not successfully read on the first attempt, then the user had to repeat the manual scan again and again.
Another feature of this invention resides in the novel optic means for focusing the highly divergent, non-radially-symmetrical laser diode beam having a generally oval beam cross-section. Advantageously, the optic means comprises a focusing lens, e.g. a plano-convex lens, and an aperture stop located in the first optical path adjacent the focusing lens. The aperture stop may have a circular, rectangular or oval cross-section which is smaller than the beam cross-section at the aperture stop so as to permit a portion of the incident laser diode beam to pass through the aperture stop en route to the symbol. The walls bounding the aperture stop obstruct and prevent the remaining portion of the incident laser diode beam from passing through the aperture stop en route to the symbol. Such beam aperturing is in direct contrast to prior art designs, such as disclosed in U.S. Pat. No. 4,409,470, wherein the incident laser diode beam is deliberately permitted to travel unobstructedly through an aperture en route to the symbol. Such beam aperturing reduces the numerical aperture from large values on the order of 0.15 to 0.45 to a value below 0.05 and significantly decreases the optical magnification factor so that a single focusing lens can be used to achieve the aforementioned advantages associated therewith. Although such beam aperturing is at the expense of output power of the laser diode, the advantages achieved are well worth such expense, and sufficient output power remains in the portion of the incident laser diode beam that passes through the aperture stop to read the symbol.
Although the use of aperture stops is well known in optical systems, such beam aperturing is believed to be novel and unobvious in laser scanning systems for reading symbols. As previously mentioned, an aperture stop decreases the power in the portion of the incident laser diode beam that impinges the symbol and, as a general rule, a laser scanning system designer does not deliberately want to throw away power, particularly in that portion of the incident beam that impinges and scans the symbol, since less power is contained in the laser light reflected off and collected from the symbol.
In addition, it is well known that for a given beam cross-section, i.e. spot size, of the incident laser beam, the depth of focus in an optical system having an aperture stop will be less than that for an optical system which does not have an aperture stop. Since, as a general rule, a laser scanning system designer wants as large a depth of focus as possible so that the working distance is correspondingly as large as possible the use of an aperture stop is something to be avoided.
It is also well known that the smallest laser beam spot size that can be theoretically obtained in an optical system having an aperture stop will be larger than that for an optical system which does not have an aperture stop. Hence, for those applications where a very small beam spot size is desired, one would not turn to using an aperture stop.
In an optical laser system which does not have an aperture stop, the laser beam spot cross-section has a gaussian brightness distribution characteristic. By contrast, when an aperture stop is employed, light diffraction causes light rings or fringes in the beam spot. Such light rings or fringes effectively cause the beam spot size to increase, as well as other undesirable effects. The undesirably increased beam spot size is still another reason why an aperture stop is not used in laser scanning systems.
On this latter point, the use of an aperture stop dictates that complex mathematics in accordance with general diffraction theory be employed to design the optical system. Since it is more often the case that laser scanning system designers work with gaussian beam mathematics, which is simpler than diffraction mathematics, this represents still another possible reason why the use of an aperture stop in a laser scanning system has not heretofore been proposed.
Rather than using an aperture stop in which the walls bounding a central aperture are opaque and block a portion of the incident laser diode beam, still another aspect of this invention resides in making the walls bounding the aperture light-transmissive. The portion of the incident beam that is transmitted through the aperture can be directed to the symbol to illuminate an aiming region thereon, or can be directed to a black body for absorption, or can simply not be used whatsoever. In any event, rather than blocking and not using the heretofore blocked portion of the incident beam, this invention proposes the use of a novel laser focusing aperture in which non-opaque walls bound the aperture and enable the portion of the beam that passes through the non-opaque walls to be used affirmatively and effectively, for example, for aiming purposes.
A particularly compact optical folded path assembly is achieved when an optical element such as a so-called xe2x80x9ccold mirrorxe2x80x9d is utilized to reflect the visible aiming light beam to a collecting mirror of the sensor means, but to transmit therethrough the reflected laser diode light reflected by the symbol and collected by the collecting mirror. Still another efficient aspect of the overall optical assembly is to integrate the collecting mirror for the reflected laser light, together with the aforementioned scanning mirror for the incident laser diode beam, as well as with the aforementioned focusing mirror for the aiming light beam into a multi-purpose mirror of one-piece construction.
Another highly desirable feature is embodied in an interchangeable component design for the head so that a manufacturer can readily adapt the head to suit the particular requirements of each user. Thus, different components may be contained in a single handle for the head, or in a plurality of interchangeable handles for the head, thereby readily adapting the head to suit the user and eliminating the laborious custom-made heads of the prior art.
Still another advantageous feature resides in eliminating the mounting of a discrete window on the head, and preventing the possibility that such a window could become disengaged from its mounting and expose the interior of the head to moisture, dust and other contaminants which could, under certain conditions, affect the operation of the head. To this end, at least a portion of the head is made of a one-piece transparent material construction, and a cover of light-blocking material is arranged over the transparent portions of the head to block light from passing therethrough, but leaving other transparent portions of the head uncovered so that said other transparent portions may serve as the aforementioned window. It is further desirable to constitute the cover of a thick, cushionable, yieldable material such as rubber to provide a measure of shock resistance for the head.
The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, best will be understood from the following description of specific embodiments when read in connection with the accompanying drawings.